Dc arc plasmatron and method of using the same

ABSTRACT

The present invention concerns a DC arc plasmatron which is a plasma generator at the atmospheric pressure having a structure suitable for improving chemical activity of plasma. 
     The DC arc plasmatron according to the invention comprises a rod cathode  110  connected to a DC power supply  1000 , and formed to be positioned close to a nozzle to be a negative; a nozzle anode  120  formed on the nozzle to be a positive, being an opening to inject plasma into a functional gap  150  and having an anode inlet  122 , wherein the plasma is created by gas discharge for forming the plasma between the cathode and the anode; a functional anode gap formed on the nozzle to be a positive so that either a function gas or a gas mixture is supplied on some part of the passage of the anode to make it react with the plasma forming gas through the nozzle inlet; and a gas supply unit for supplying a plasma forming gas flowing through the nozzle anode from the side of the rod cathode. 
     The anode comprises an anode inlet  122  as an inlet of the functional gap  150 ; an anode outlet  125  as an outlet of the functional gap  150 ; the functional gap  150  formed as a disk shaped space  300  communicating with a part of the passage between the anode inlet  122  and the anode outlet  125 ; a gas collecting chamber  500  formed with a chamber member communicating with the disk shaped space  300 ; and a functional process gas conduit  600  communicating with the gas collecting chamber  500  and formed so that either the functional gas or a gas mixture can be supplied. 
     The functional gap  150  is formed to have a height G to be 0.3A to 3A with respect to the diameter A of the anode inlet  122.    
     The anode outlet  125  is formed to have a diameter D to be 0.3A to 3A with respect to the diameter A of the anode inlet  122.    
     A method of using the plasmatron according to the invention comprises the steps of: a first step of selecting a proper functional gas, supplying it to the functional process gas conduit  600  and thereafter supplying a plasma forming gas in the space around the rod cathode  110  and the anode  120 ; a second step of supplying cold water and operating the DC power supply  1000  connected to the plasmatron; a third step of initiating arc discharge by applying high voltage across the rod cathode  110  and the anode  122 , and activating the plasma forming gas and the functional gas by interaction therebetween while the plasma forming gas is excited and heated; and a fourth step of, after fixing a process variable to a proper level, discharging the gas in the arc discharge area through the anode outlet  125  of the plasmatron to reach a appropriate vacuum pressure while keeping the process variable at an appropriate level. 
     Chemical activity of plasma jets formed by means of the plasmatron and the method according to the invention is improved and the completion level in the chemical process of plasma by the functional gas is improved.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention relates to a DC arc plasmatron and to a method of using the same.

More particularly, the invention relates to a DC arc plasmatron comprising a rod cathode connected to a DC power supply, and formed to be positioned close to a nozzle to be a negative, a nozzle anode having an anode inlet formed on a nozzle, being an opening to inject plasma created by gas discharge for forming plasma in the space between the cathode and the anode and functioning as an anode inlet, a functional gap formed to be a positive so that either a functional gas or a gas mixture flows in on some part of the passage of the anode to make it react with the plasma forming gas through the anode inlet, and a gas supply unit for supplying the plasma forming gas flowing through the anode inlet from the side of the rod cathode. The inventive DC arc plasmatron is structured to be a shape suitable for improving chemical activity of plasma and is a plasma generator at the atmospheric pressure.

The invention also concerns a plasma generator for heating an operating gas by means of DC arc discharge at the atmospheric pressure or at a pressure reduced state, and can be applied to many engineering fields of electronics, metal, vehicles and the like, for example, for purification of exhaust gas, in a plasma-surface process for surfaces treatment such as washing, corrosion resistance, deposition, etc. to water purification and ozone treatment, incineration of waste, sterilization and disinfection of medical instruments and living matters for forming protective membranes on injuries, etc.

2. Description of the Prior Art

There has been widely known a plasmatron having a bar cathode, a nozzle anode, means for supplying gas for forming plasma in the space between the electrodes, a power supply unit and an initiator.

The electrodes of the plasmatron made of a material enduring high temperature, for example, tungsten, discharge hot electrons when heated to a temperature of 2,500° C. or higher and have a corrosion rate of 10⁻⁹ g/° C. or so. Such a low corrosion rate of tungsten only holds for plasma forming gases chemically inert such as argon or nitrogen.

A water-cooling anode is more severely damaged at a pressure around the atmospheric pressure. In particular, in case of copper generally used as a material for producing plasmatrons, it has a corrosion rate of 10⁻⁶ g/° C. or higher, which is four orders higher than that of the cathode made of, for example, tungsten. Therefore, it is the anode that limits the service life of such a plasmatron. The service life thereof generally ranges approximately from several hours to tens of hours. The average temperature of plasma (3000-6000K at a free arc, 5000-25000K at a stabilized arc), relatively high, causes much pollutant such as copper vapor, masses, ions, etc. to be generated which prevents the plasmatron from being applied to various engineering fields.

There is also known a similar DC arc plasmatron specially designed to reduce an anode corrosion rate and pollution levels of plasma by attaching an insertion unit made of refractory metal to the wall of the anode opening, and to prolong the service life of the plasmatron. This special plasmatron has been proposed to be used to modify hydrocarbon fuel by means of the chemical process of plasma. In this apparatus, fuel is supplied from a ring-shaped collector surrounding the anode opening communicating with the fuel supply conduit to a space of a plurality of openings arranged in an annular form on the lower part of the insertion unit.

As a result, with this apparatus, interesting fuel modification could be obtained. The energy efficiency by the aforementioned technique, however, was not so high, due to a low excitation capability of arc plasma. The reason is presumed to be non-uniform heating of the plasma forming gas in the anode spot area. In case of using an insertion unit made of refractory metal, the anode spot is also limited to a very small area (the process associated with the anode spot locally existing involves local melting, overheating and implosion). Accordingly, only a very small part of gas flow contacts the heated anode spot area, and it is thus impossible to reach an average temperature sufficiently high. Such a phenomenon is an obstacle to completing the chemical process of plasma and lowers a reaction product yield of the plasmatron.

There is known a method of actually using the aforementioned plasmatron, comprising the steps of: selecting a suitable functional gas; supplying a plasma forming gas; supplying cold water and operating the power supply connected to the plasmatron; initiating arc discharge between an anode and a cathode; thereafter exciting and heating the plasma forming gas; and activating the functional gas by means of the reactions between the plasma forming gas and the functional gas.

When operation of the plasmatron at a vacuum is intended in order to raise the plasma ionization level and the gas activation level, the aforementioned method cannot be applied. When the pressure drops under a critical point, discharge at a farther distance is easier according to the Paschen's Law. Arc initiation at a vacuum thus results in dielectric breakdown in the space between electrodes. In this case, it is impossible to avoid problems by the process that very active arc spots appear both on the anode and the cathode, which contact a dielectric or soldered part that may be easily broken.

BRIEF SUMMARY OF THE INVENTION

In addition to the aforementioned process, the invention includes a method of performing the aforementioned process at the atmospheric pressure in the arc area, and discharging the arc area through the nozzle of the plasmatron after fixing a process variable to an appropriate level to achieve an appropriate vacuum pressure while keeping the process variable. In this case, the arc can be ignited through a shortest distance and the plasmatron according to the invention has the same service life as that at the atmospheric pressure.

It is an object of the invention to provide a DC arc plasmatron for improving excitation efficiency of a process gas and for enhancing the strength and the completion level of chemical reaction of plasma carried out by means of the plasmatron.

It is another object of the invention to provide a DC arc plasmatron for prolonging the service life thereof and improving the cleanliness level of created plasma.

It is still another object of the invention to provide a DC arc plasmatron for controlling the shape and size of discharged plasma jets in conformity with a specified purpose.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will become apparent from the following detailed description of preferred embodiments thereof illustrated with reference to the accompanying drawings, wherein:

FIG. 1 shows schematically components of a DC arc plasmatron according to the invention, of a rod cathode 110, an anode 120, a anode inlet 122, a functional gap 150, a disk shaped space 300, a gas collecting chamber 500, and a functional process gas conduit 600, in a top-to-bottom cross section of the DC arc plasmatron cut vertically;

FIG. 2 is a cross section of the plasmatron in FIG. 1 in the A-A direction, showing the components of the plasmatron according to the invention, comprising the gas collecting chamber 500, and the functional process gas conduit 600; and

FIGS. 3 to 7 are cross sections showing exemplary variations of the disk shaped space 300 and the gas collecting chamber 500 in FIG. 1

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention pertains to a DC arc plasmatron.

The DC arc plasmatron according to the invention comprises a rod cathode 110, a nozzle anode 120 and a gas supply unit.

More particularly, the invention relates to a DC arc plasmatron comprising a rod cathode connected to a DC power supply, and formed to be positioned close to a nozzle to be a negative, a nozzle anode formed on the nozzle to be a positive, being an opening to inject into a functional gap 150 plasma created by gas discharge for forming the plasma between the cathode and the anode and having an anode inlet 122, a functional gap so that either a function gas or a gas mixture is supplied on some part of the passage around the anode to make it react with the plasma forming gas through the anode inlet 122, and a gas supply unit for supplying a plasma forming gas flowing through the space around the nozzle anode from the side of the rod cathode.

Now, embodiments of the invention will be described in detail with reference to the accompanying drawings.

FIG. 1 shows schematically components of a DC arc plasmatron according to the invention, of a rod cathode 110, an nozzle anode 120, an anode inlet 122, a functional gap 150, a disk shaped space 300, a gas collecting chamber 500, and a functional process gas conduit 600, in a top-to-bottom cross section of the DC arc plasmatron cut vertically.

FIG. 2 is a cross section of the plasmatron in the A-A direction in FIG. 1, showing the components of the plasmatron according to the invention, comprising the gas-collecting chamber 500, and the functional process gas conduit 600.

FIGS. 3 to 7 are cross sections showing exemplary variations of the disk shaped space 300 and the gas collecting chamber 500 in FIG. 1.

The DC arc plasmatron according to the invention comprises a rod cathode 110 connected to a DC power supply 1000, and formed to be positioned close to a nozzle to be a negative, a nozzle anode 120 formed on the nozzle to be a positive, being an opening to inject into a functional gap 150 plasma created by gas discharge for forming the plasma between the cathode and the anode and having an anode inlet 122, a functional gap 150 so that either a function gas or a gas mixture can be supplied on some part of the passage around the anode to make it react with the plasma forming gas through the anode inlet 122, and a gas supply unit for supplying a plasma forming gas flowing through the nozzle anode from the side of the rod cathode.

The anode 120 comprises: an anode inlet 122 as an inlet of the functional gap 150; an anode outlet 125 as an outlet of the functional gap 150; the functional gap 150 formed as a disk shaped space 300 communicating with a part of the passage between the anode inlet 122 and the anode outlet 125; a gas collecting chamber 500 formed with a chamber member communicating with the disk shaped space 300; and a functional process gas conduit 600 communicating with the gas collecting chamber 500 and formed so that either the functional gas or a gas mixture can be supplied.

The functional gap 150 is formed to have a height G to be 0.3A to 3A with respect to the diameter A of the anode inlet 122.

If the value G is not within the range, the ratio for mixing the plasma forming gas and the functional gas cannot be within a proper range, either, so that it is impossible to raise the strength and the completion level of the chemical reaction of plasma.

The anode outlet 125 is formed to have a diameter D to be 0.3A to 3A with respect to the diameter A of the anode inlet 122.

Where the value D is larger than 3, the time at which the functional gas stays in the functional gap is too short and sufficient reaction cannot thus be achieved. Where the value D is smaller than 0.3, it is hard to form effective vortexes, so that it is impossible to enhance the strength and the completion level of the chemical reaction of plasma.

The functional gap 150, the disk shaped space 300 directly communicating therewith, the gas collecting chamber 500 and the functional process gas conduit 600 in the DC arc plasmatron according to the invention may be integrated as an vortex generator 800.

Also, the functional process gas conduit 600 which forms a part of the vortex generator 800 may be formed as inlet openings in the tangent direction to the circumference of the annular gas-collecting chamber 500.

Also, the functional process gas conduit 600 which forms a part of the vortex generator 800 may be formed as spiral openings.

A method of using the DC arc plasmatron according to the invention comprises the steps of: a first step of selecting a proper functional gas, supplying it to the functional process gas conduit 600 and thereafter supplying a plasma forming gas to the space around the rod cathode 110 and the anode 120; a second step of supplying cold water and feeding power from the DC power supply 1000 connected to the plasmatron; a third step of initiating arc discharge by applying a high voltage across the rod cathode 110 and the nozzle anode 120, and activating the plasma forming gas and the functional gas by interaction therebetween while the plasma forming gas is excited and heated; and a fourth step of, after fixing a process variable to a proper level, discharging the gas in the arc discharge area through the anode outlet 125 of the plasmatron in order to reach an appropriate vacuum pressure while keeping the process variable at an appropriate level.

In more detail, unlike the previous invention (a DC arc plasmatron having an anode corrosion rate of about 10⁻¹⁰ g/° C.) by the inventors, in the plasmatron according to the present invention, the functional gas reacts with the plasma forming gas in the functional gap and changes its flowing direction by 90° to be discharged through the anode outlet. Such a structure has an effect to enhance reaction activity.

In the plasmatron according to the invention, the functional process gas conduit 600 consists of one or more slits or circular openings annularly arranged. In order to control the shape and the size of plasma jets discharged in conformity with a special purpose, the size of the functional gap 150 can be made to be smaller or larger than the diameter of the anode inlet122 opening.

Also, in order to control the shape and the size of plasma jets discharged in conformity with a special purpose, the diameter of the anode outlet 125 can be made to be smaller or larger than the diameter of the anode inlet 122. The size G of the functional gap 150 and the diameter D of the anode outlet 125 are determined to be G=(0.3˜3)A with respect to the diameter A of the anode opening and D=(0.3˜3)A according to the range by means of the aforementioned formula.

Also, to the functional gap 150 is connected the vortex generator through the gas collecting chamber 500. The vortex generator 800 consists of an inlet of the functional process gas conduit 600 produced as an inlet opening or spiral screw shaped opening of the functional process gas conduit 600 in the tangent direction to the circumference of the collector and the disk shaped gap, a gas collecting chamber 500 and a disk shaped space 300.

In order to assist activation of the functional gas, the internal surface forming the functional gap 150 and the disk shaped space 300 may be formed to be flat, funneled or a combination of both shapes so that the direction of the functional gas flow may be normal or have a given angle to the axial flow of the plasma forming gas.

The DC arc plasmatron according to the invention basically comprises an anode 120, a rod cathode 110 and an inter-electrode dielectric not shown in FIG. 1. Both of the electrodes, the anode and the cathode, are cooled with water.

The functional gas flows to meet the heated plasma forming gas in the arc area. By means of the reaction between the two gases flow, the functional gas is heated and the chemical reaction of plasma thereby occurs. The electron temperature in the dispersed anode point is considered to be even higher than that of heavy particles consisting of plasma at the atmospheric pressure as well as at a vacuum. Therefore, the plasmatron according to the invention can induce chemical reaction of plasma, very unique and interesting in practice.

The DC arc plasmatron according to the invention exhibited a very low anode corrosion rate through testing, which means that the anode point is well dispersed and distributed. When the height G of the functional gap and the diameter D of the anode outlet belong to the range of 0.3A≦(G, D)≦3A with respect to the diameter A of the anode opening, such a dispersed distribution of the anode point was kept in various test conditions. The anode point distributed wide lowers current density in the anode part by several units and lowers the anode corrosion rate to 10⁻¹⁰ g/° C.

The arrangement of the functional process gas conduit 600 as shown in FIG. 2 affects the gas reaction state. Vortex forming may be achieved by means of the inlet formed as a spiral screw shaped opening communicating with the collector.

FIGS. 3 to 7 show variations of the gap and the opening, which is flat, funneled or a combination of both shapes, in a cross section. The functional gas in FIGS. 3 and 5 flows into the same direction as that of the plasma forming gas. In FIGS. 4 and 6, the functional gas flows in the direction against the plasma forming gas flow. In FIG. 7, the functional gas flows in the direction perpendicular to the plasma forming gas flow on the anode axis.

In the anode channel structure of the DC arc plasmatron according to the invention, swirls are made in the area where the functional gap 150 meets the anode opening. As well known in arc discharge physics, the anode point is drawn on the anode surface by means of the swirls. Such an anode point drawn by means of swirls allows the anode point to be dispersed and distributed on an even wider circular area than in a limited area in prior art. The anode point distributed wide lowers current density of the anode part by several units and also lowers the anode corrosion rate to 10⁻¹⁰ g/° C. or so. In the condition of the dispersed anode point, since, if tungsten is used which operates in the thermo-ionic mode, the cathode corrosion rate is 10⁻⁹ g/° C., the service life of the plasmatron and the pollution level of plasma are determined by means of the cathode corrosion process that has an corrosion rate one order higher than the anode corrosion rate. In such a condition, the service life of the plasmatron can be prolonged to be thousands of hours and such a long service life is sufficient to be applied to an actual industrial site. In the DC arc plasmatron according to the invention, the reaction between the functional gas and arc plasma is even stronger as compared to that by conventional apparatuses. The reason of such improvement is that the anode point is distributed uniformly wide, thereby contacts the gas flowing into the entire area of the gap 150 and is heated uniformly by the swirls. Also, the functional gas reacts with the whole part of the positive column of arc after passing through the gap area. As a result, in the plasmatron according to the invention, the efficiency of the chemical process of plasma and the chemical excitation level of the functional gas is significantly improved.

The DC arc plasmatron according to the invention is operated according to the following steps: operating all parts of the gas supply system and the cooling system, supplying power and then pressing the initiation button; after arc discharge starts, controlling currents to a necessary level; controlling the gas flows into a desired technical target; after carrying out desired processes, switching off arc power supply; switching off all parts in the reverse order to the initiation process and then shutting off cold water supply.

From the foregoing description, it will be observed that various modifications and changes can be made without departing from the true sprit and scope of the present invention. It should be understood that the foregoing description is intended to illustrate and not to limit the scope of the invention as defined by the claims. 

1. A DC arc plasmatron comprising a rod cathode, a nozzle anode having a body member and a through axial orifice, a power supply unit connected to both of the electrodes, and a gas system for feeding plasma-forming gas into the inter-electrode space and supplying suitably selected technologic gas or gas mixture into the anode orifice through an internal opening communicating with said orifice and positioned between its inlet and outlet parts, wherein said opening is configured as a continuous circular axial gap between said parts of said anode orifice, while the size of said gap can be smaller or larger than the diameter of the inlet part of said anode orifice and the diameter of the outlet part of said anode orifice can also be smaller or larger than the inlet, part of said anode orifice.
 2. A DC arc plasmatron as claimed in claim 1, wherein said anode is configured such that the ratio of the size G of said technologic gap to the diameter A of the inlet part of said anode orifice is between 0.3 to
 3. 3. A DC are plasmatron as claimed in claim 1, wherein said anode is configured such that the ratio of the diameters D to A of the outlet and inlet parts of said anode orifice respectively is between 0.3 to
 3. 4. A DC arc plasmatron as claimed in claim 1, wherein said circular gap is formed as a vortex injector.
 5. A DC arc plasmatron as claimed in claim 1, wherein said vortex injector has tangent inlet holes.
 6. A DC arc plasmatron as claimed in claim 1, wherein said vortex injector has a supplying auger at its inlet.
 7. A DC arc plasmatron as claimed in claim 1, wherein internal surfaces of said gap are shaped flat, conical or a combination of both of said shapes to direct technologic gas normal or at some angle to the axis, thus providing wake or cross flow of technologic gas with respect to the axial movement of the plasma-forming gas.
 8. A method of using said DC arc plasmatron as claimed in claim 1, comprising the steps of: selecting, suitable technologic gas; switching on water-cooling and gas-supplying systems of the plasmatron; and exciting arc discharge between its anode and cathode, wherein the above steps are carried out at the atmospheric pressure in the arc area, then process parameters are adjusted to a given level and said area is evacuated through the anode orifice while keeping the above process parameters until suitable vacuum pressure in the treatment zone is reached. 